Arrangement for measuring frequency characteristics



P. DESERNO 2,356,510

ARRANGEMENT FOR MEASURING FREQUENCY CHARACTERISTICS Aug. 22, 1:.

Filed Sept. 6, 1940 APPA RA 7U! UNDER TES Y Fig.3

device under test Patented Aug. 22, 1944 I ARRANGEMENT FOR MEASURINGFRE- QUENCY CHARACTERISTICS Peter Deserno, Berlin, Germany; vested inthe Alien Property Custodian Application September 6, 1940, Serial No.355,599

In Germany September 9, 1939 4 Claims.

This invention relates to devices for testing the frequencycharacteristic ofelectrical devices and consists in certain features ofnovelty which are described hereafter in connection with the drawing, inwhich Fig. 1 shows oscillographic indications referred to in explainingprior arrangements; Fig. 2 is a graph that serves to explain thefunction of another kind of prior device and also serves to disclose theidea of the present invention; Fig. 3 is a diagram representing oneembodiment of the invention; and Fig. 4 is a fragmentary diagram showinga modification of the arrangement illustrated in Fig. 3.

Fig. 5 is a diagram indicating the wave forms of the currents throughrectifying tube R1, deflecting coils LA and LR and device J.

Many electrical devices manufactured in large quantities are tested andadjusted by observing their frequency characteristics. The testingdevices used usually record the frequency characteristic directly uponthe screen of a cathode ray tube, by employing to this end aperiodically,

varying frequency potential supplied by the testing set. This setapplies to the sweep circuit deflectors of the cathode ray tube apotential which varies in magnitude synchronously with the vari-- ationof frequency. Further, this set applies to the input side of the deviceto be tested a potential which has the predetermined frequency variationbut substantially constant amplitude. The

instantaneous output potential or current of this plied frequency,acteristic. being tested is applied to the test circuit deflecaccordingto its frequency char- This output potential of the device varies withthe variation in aptors of the tube, to afford a test deflectionperpendicular to the sweep deflection, to present on the screen of thecathode ray tube a visible image of the frequency characteristic of thedevice under test.

In such testing devices the sweep or time base axis of the cathode raytube is usually marked with indications designating the respective fre;quencies.,

It is desirable that increments in frequency be proportional tocorresponding increments of distance along the time base axis, so as torepresent observed characteristics as conventionally graph,- ically aspossible. A frequency scale, however, that involves suchproportionality-is not easy to produce, as is explained in thefollowing.

Such a test device includesa high frequency generator or oscillator,adjustable in frequency generally by means of a variable condenser. If

the condenser which'controls this frequency, as well as the time basegenerator, has the usual sinusoidal capacity variation common in simpletest units a distorted frequency scale results. This distortion is dueto the non-linear proportionality between frequency and capacity, as canbe seen by the formula 1 f 21r ZE I As a result, the scale is crowded atone end, and the curve is not a conventional illustration of thefrequency characteristic; I It appears, therefore, that in order toproduce an undistorted curve, this distorted frequency scale must alwaysbe taken into consideration. The magnitude of distortion due to thenon-linear relationship of frequency to capacity may be, seen from Fig.1, in

which curve a is representative of a typical resonance curve on a linearscale, while b represents the same resonance curve on a distorted scale.

In other devices frequency variation may be produced by inductancevariation. For instance, the frequency of an oscillator in which thecoil of the oscillatory circuit has a core of high frequency iron, maybe greatly varied by polarizing the high frequency iron to differentdegrees. To such end the high frequency iron is preferably 1 arranged inthe iron core C of a'low frequency choke which is then more or lessefficiently energized by an alternating current or a pulsatinguni-directional current. The frequency of the high frequency generatorthen varies with the fluctuations of such energization.

Fig. 2 is a plot of percentchange in frequency against deflectioncurrent and illustrates the interdependence between the energizingcurrent and testing frequency of'an arrangement of the latter type, inwhich frequency variation is obtained from inductance control. As willbe seen,

the curve here shown is substantially rectilinear within a large region,that is to say, percent frequency variation increments are proportionalto corresponding current variation. In spite of this seeming linearity,a linear frequency scale may not be traced on the screen of the cathoderay tube, firstly because it is very dimcult, on account of the timeconstant of the coils, .to obtain arectilinear rise of current,

and then the deflecting voltage and current variation in the choke 0must be produced by electron tubes 0 and R1. The curvature of thecharacteristic curves of these tubes renders it almost screen of thecathode ray tube and the percentage frequency chang where Ci and CI areproportionality constan From these relations it'follows thatphasedistortion maybepr'esent and effective to a considerable extent.

In order to do awaywith all these sources of. error and thereby toobtain a linear frequency scale it is proposed by theinvention that'the.variation of thetesting frequency and the time base deflection of thebeam be effected by the same current. The deflection inthe direction ofthe time base is here effected magnetically, that is, in a manner notusual with testing arrangements. Y

Since the sweep or time base deflection A on of the high frequency'generator (straight portion of the curve of Fig. 2) r are bothproportional to the current, J the proposed arrangement will necessarilycause proportionality between deflection and frequency. We may thusexpress the following, relating deflection to frequency:

aswillbeclear. X U

jlnl 'lg. 3, which shows an arrangement for toeifectafrequency change.Coil'lsisa'rranged o i v guano I YI/ to avoi distortion. rneddition.

is coupledito the output of oscillator O output from thii apparatus isapplied to deflectingcoilslrrarranged'atrightahsleltqtimebase, 1deflection coils La. Indications are produced on the screen 8 by thecathode ray tube 3 indicative of the ireq'uency characteristics ofthe-apparav tus-i.

' 1"ig. 2 indicates that in order properly t utilise the current-percentfrequency characteristic, it

about which to operate, is therefore required.

In accordance with the'circuit of Pig. 8 this mean supply current valuemay be-ccntrolled by tube 31. Conceivably, the use of such pulsatingdirect current may be effective to deflect thebeam of the cathode raytube away from the middle of the screen 8 thereof. Some compensationmeans must, therefore, be provided to offset this deflec-' tion. In theform shown an additional deflection coil L: is provided for thispurposeand is so dimensioned and connected that the direct I currentflowing through it wfli beeffective to return a recorded curve to aposition generally centrally of the fluorescent screen. A potentiometerP may be provided shunting the L: winding in order to regulate thecentral location of the Obsi served image within narrow limits. Byvarying curve changing its position. In Fig. 5 there is 7 showndiagrammatically the wave forms of oscillator O, thisinductancehaving-to be varied inthegapofhlghfrequencyironcorec whichservesasthecoreformagnctisingcoilhr. The variation influnasener'gisationofcoil Lu is varied. affects coillla to produce the desired frequer'icyDeflection coils La provide sweepdeflectionofthecathoderaybeamoftubewbislPPMinthenormal-- manner between the grid andcathode of tube .."Riallowing pulsating direct current of theform showninl 'lgiitobesuppliedfromanasource through the plate-cathode circuit The directcurrent component ofthe pulsating directcurrenttraversescoilsInJnandk,

eurrentthroughlsrbeingadiustableatl'tocenter'properly, the cathoderaybeam, as is'described in more. detail-later. The alternatingcomponent oft-hecurmtdoesnottraversek but instead is by-passed-Jrycondenser ch89 that this component may serve to cause periodictimeddeflectionofthecathoderaybeam. The

' the equation hereheforededuced. The aim, of

' J, Fig. 2', indicates the currentflowing. throughcourse,shouldbetohaveafairlyconstantriseof' current in order to insurethat the indicating speed in the cathode ray hibe'and hencethebrightness of the record he likewise constant. *In

a given case the brightness 'of'the cathode ray tube might be continuedindependence on the indicating speed. The apparatus under test thedirect current amplitude of tube Bi by means of the adjustableresistance as showri rthe enflre frequency range may nowbe' alteredwithout the potentialsthroughIKandJ.

The frequency range of the-testing device is determined by the ampereturns of choke wind-1 ,1'

In order to provide for altering this range thecoil In may be flttedwith taps, as a, b.

. c, d when employingthe small ranges bd or c-d it is consideredpreferable to replace the 'remai'njder of coil I-sr y a separateinductance connectedbetween the taps a, b or a, 0, respec-- tively. Thispreferred embodiment is shown in.

Pig. 4 in which deflection potentials may be derived across taps b andc,'and a separate inductance D is added between terminals a' and b.

In order to adJustthe circuit described to adesiredworkingpoinathecoremsayinthe Fig. 4 arrangement, maybeisuitably'polariaed bya biasingdirectcurrent suppliedtohiasing,windingLv.,-Itisclearthatappropriatecontrol of this biasing directcurrent will suitably adjust the working point, and furthermore, thatsuch,

adjustmentmayalsoservetoshiftthefrequeney rangeofthetestingdevice.

J,I 'ig.3,denotesameasuringinstrument,

Whatisclaimedis:

1. A testing devicefor recording electrical irequency characteristics ofa circuit onthe screen of a cathode ray tube by deflection of the beamthereof, comprising source of periodically vary; ing testing frequencyfor application to the'cir-v cult to be tested, beam deflecting meansfor connection to the circuit'under test, beam deflecting coils forcausing a time base deflection ofsaidbeamatanangletothete'stingdeflection, said source including acontrol coil for controlling the periodic variations of said testingfrequency,

and pulsating direct current supply means, said beam deflecting coilsand said control coil being connected in series with each other and withsaid pulsating direct current supply means whereby the variations insaid testing frequency and said time base deflections are effected bythe same current.

2. A device according to claim 1, wherein said source of the saidtesting frequency includes a coil, taps on said coil, and meansconnecting the 5 current supply source and associated elementsadjustably between certain of said taps, whereby the range of testingfrequency may be changed.

3. A device according to claim 1, wherein the means for controlling saidtesting frequency furl0

